Dry winters delay breeding for desert-dwelling birds

You’d think that animals adapted to the driest of deserts would get along just fine without much water. While deserts may typically be hot and dry, wildlife communities have evolved to rely on the brief rains that blanket their communities during the so-called “wet seasons.” Despite their desert lifestyles, severe droughts – like the one plaguing California in recent years – make survival even tougher for the desert dwellers.

Just how does rainfall (or the lack thereof) affect desert critters? To find out, University of Arizona researcher Chris McCreedy teamed up with USGS scientist Charles van Riper III, and they turned their attention to the birds. In particular, they focused on thirteen bird species nesting in two spots in the Sonoran Desert, one in California and one in Arizona.

From an ecological perspective, the wash systems of the Sonoran Desert are similar to other riparian zones, but surface water is only present during the heaviest of heavy rains. Winter rains fall from November through April and result from cyclones that derive from Pacific storm tracks. Summer rains occur from July through September, and are the result of convective thunderstorms. Winter rains tend to drop lots of water, but gently and over a prolonged period of time; summer thunderstorms are more severe and more localized, but much shorter-lived. The two wet seasons are punctuated by extremely dry autumns and springs.

The researchers collected historical rainfall data for the years 2004-2009 and matched it with data on the first egg laid for each breeding pair of the thirteen species they had monitored. The mean rainfall for each of the study sites was identical, with around 55mm of average annual rainfall (just over two inches). The droughts of 2006 and 2007 saw less than 10% of that amount, while the 2005 El Niño year brought 500% of the average rainfall.

They discovered a distinct negative relationship between winter rainfall and the date on which the nesting season began. In other words, the less wet the winter was, the later the birds began laying eggs. The rainier it had been, the easier the birds began to breed. That pattern held up for each of the thirteen species the researchers considered, which each had to have at least four nests each season. And they suspected that the later egg-laying meant for a reduction in reproductive success. What’s perhaps puzzling is that, elsewhere in North America and Europe, changing climates are associated with earlier nesting, not later. What’s going on?

To further understand what was going on, McCreedy and van Riper conducted a true experiment. The winter of 2010 included a fair amount of rainfall, at least for the Sonoran desert. It wouldn’t have been classified as a particularly dry year. So the researchers artifically induced delayed egg laying in two species. They pushed the first eggs of black-tailed gnatcatchers by two weeks, and of verdin by three weeks, to match the delayed nesting that they had observed in the observational part of the study. They were able to delay the egg-laying by discretely poking holes into the birds’ nests. As long as they kept creating holes, the birds would keep on patching them, delaying their nesting until the nest was sufficiently stable. It took fewer than five seconds for a researcher to create the hole, while the birds required 2-3 days to fix it. Then, 5-7 days before the target egg-laying date, the researchers finally left the birds alone.

As the researchers expected, the delayed egg-laying had negative consequences for the birds’ nesting success and reproductive productivity. So not only did dry winters lead to later breeding, but later breeding was far less productive than earlier breeding. “Treatment pairs were unable to compensate for these missed nesting opportunities due to low nest survival rates over the remainder of the nesting season,” write the researchers. Why might that be? Why are wet winters to crucial to the birds’ successful reproduction?

It turns out that eggs laid late in the breeding season are more susceptible to predation both by other birds and by snakes, and clutches laid late in the season are also more susceptible to brood parasitism by the brown-headed cowbird.

If the researchers are correct, then the reduced rainfall means there’s less for the birds to eat, and that means they’ve not gained enough nutrients to lay eggs until late in the season, and those eggs simply become less likely to successfully hatch.

The combined pressures of predation and parasitism mean that the birds of the Sonoran Desert are best served by breeding earlier in the season. But thanks to reduced wintertime precipitation, those birds are being forced to lay their eggs at suboptimal times. “To understand how late the delay is, it would be like if the robins nesting in your yard, who typically begin nesting in late April, did not begin to nest until nearly Memorial Day,” said McCreedy in a statement.

Unless the Sonoran and Mojave Deserts of the American Southwest can find their way out of the current drought and the even drier conditions predicted for the coming years, the birds’ future remains uncertain. Birds are already among the world’s most threatened groups of animals, and according to the 2014 National State of the Birds Report, birds in arid landscapes showed the steepest declines of all. – Jason G. Goldman | 07 January 2015

Source: McCreedy C. & van Riper III, C. (2014). Drought-caused delay in nesting of Sonoran Desert birds and its facilitation of parasite- and predator-mediated variation in reproductive success, The Auk, 132 (1) 235-247. DOI: http://dx.doi.org/10.1642/auk-13-253.1

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